1. Field of the Invention
The present invention relates to a method for water-cooling metallic slabs and, more particularly, to a method for cooling steel slabs by dipping them in water while they are still at a high temperature, after continuous casting. The present invention relates also to an apparatus suitable for performing this method.
2. Description of the Related Art
It is common practice in steel production that refined molten steel of a desired composition is made into slabs by continuous casting or ingot making. Subsequently they are made into steel products of desired shape by hot rolling or cold rolling. Slabs are sometimes cooled in water while they are still hot after solidification. This cooling is intended to avoid transformation which would otherwise aggravate the surface and internal quality of steel products and also to avoid undesirable precipitation.
A problem that arises when continuously cast stainless steel slabs are allowed to cool spontaneously is that alloying elements (such as chromium) in the steel combines with carbon to form carbides which selectively precipitate at grain boundaries, thereby forming a chromium-deficient layer in the vicinity of the precipitates. The result of rolling such slabs containing an uneven composition, particularly in the case where hot rolling is followed by cold rolling, is development of surface defects such as irregular gloss.
In addition, continuously cast slabs are subject to cyclic surface irregularities (oscillation marks) due to vertical oscillation of the mold. Such surface irregularities have troughs in which nickel segregates. This leads to grain-like defects after rolling and pickling.
In order to address the above-mentioned problem, the present inventors had previously proposed a process for producing stainless steel slabs (Japanese Patent Laid-open No. 87054/1994) and a process for refining stainless steel slabs (Japanese Patent Laid-open No. 266416/1992). The former is characterized by cooling cast slabs continuously at a cooling rate higher than prescribed. The latter is characterized by cooling cast slabs continuously (with the surface temperature kept higher than 400.degree. C.), performing shot blasting, heating to 1100.degree. C. and above, and removing scale from slabs. The present inventors had also proposed an apparatus for cooling hot slabs in water (Japanese Patent Laid-open No. 100609/1995).
The processes and apparatus mentioned above, however, were found to cause surface defects (such as uneven gloss and scab) when applied to the production of stainless steel sheet from continuously cast stainless steel slabs by hot rolling and cold rolling.